A gradient perovskite heterojunction and a preparation method and application thereof

Abstract

The present application relates to the technical field of X-ray detection, and particularly relates to a gradient perovskite heterojunction and a preparation method and application thereof. The present application provides a preparation method of a gradient perovskite heterojunction, comprising the following steps: using an electric field driven ion deposition method, under the condition of applying a reverse bias electric field, ITO glass and ITO glass fixed with a perovskite single crystal are relatively arranged in an AX ethanol solution to perform first crystallization, a perovskite gradient layer is obtained, then the reverse bias electric field is switched to a forward bias electric field to perform second crystallization, and the gradient perovskite heterojunction is obtained; A in AX is an organic cation or an inorganic cation, and X in AX is a halide anion. The preparation method can prepare a gradient perovskite heterojunction for constructing a compact integration, and has the advantages of rapidness, strong universality and high adhesion.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of X-ray detection technology, in particular to a gradient perovskite heterojunction and a preparation method and application thereof. BACKGROUND

[0002] Perovskite semiconductors have become excellent candidates for the next generation of X-ray detectors due to their unique tunable composition, high carrier mobility and lifetime product (μτ), high radiation attenuation coefficient or optical absorption coefficient, and low defect density. Three-dimensional (3D) perovskites, such as MAPbI3 and FAPbI3, have been shown to have excellent performance with high sensitivity of about 1 x 10 3 ~ 1 x 10 6 μC·Gy -1 ·cm -2 However, under the action of an electric field, high dark current, noise, and significant ion migration cause stability problems. In contrast, two-dimensional (2D) perovskites not only have extremely high bulk resistance that can suppress dark current and noise, but also exhibit inhibition of ion migration. However, due to the long-chain spacer cations in two-dimensional perovskites, the charge is hindered, resulting in lower sensitivity. Therefore, the hetero-engineering of three-dimensional / two-dimensional perovskites, which organically combines the advantages of different components, has rapidly become a research hotspot this year.

[0003] Among the many studies on how to effectively form a heterostructure, liquid phase epitaxy is one of the most effective and common methods. It allows two or more single-crystal perovskites to be firmly combined to form a heterostructure. However, such a method still has limitations: 1) although spatial confinement techniques can be adjusted, it is still challenging to grow a specified perovskite at a specified location, and the versatility is poor; 2) the interface between different perovskites is too obvious, which cannot effectively form a gradient heterostructure, and is not conducive to the construction of a built-in electric field with a wider depletion layer. It is worth noting that a planar heterojunction with a transition layer thickness of less than 1 μm can still extract photo-generated charges within an effective distance, but a planar heterojunction with a sharp interface is still not the best choice for X-ray detection. Therefore, it is still challenging to construct a tightly integrated gradient perovskite heterojunction to improve X-ray detection. SUMMARY

[0004] The present application aims to provide a gradient perovskite heterojunction and a preparation method and application thereof, which can prepare a tightly integrated gradient perovskite heterojunction, and has the advantages of rapidity, strong versatility, and high adhesion.

[0005] In order to achieve the above-mentioned application purposes, the present application provides the following technical solutions:

[0006] The present application provides a preparation method of a gradient perovskite heterojunction, comprising the following steps:

[0007] After the first crystallization, the ITO glass and the ITO glass fixed with perovskite single crystals are oppositely arranged in the AX ethanol solution, and then a reverse bias electric field is applied to obtain a perovskite gradient layer, and then the reverse bias electric field is switched to a forward bias electric field to perform the second crystallization to obtain the gradient perovskite heterojunction.

[0008] The A in the AX is an organic cation or an inorganic cation, and the X in the AX is a halide anion.

[0009] Preferably, the organic cation is MA + and / or FA + .

[0010] The inorganic cation is Cs + .

[0011] Preferably, the perovskite single crystals in the ITO glass fixed with perovskite single crystals are two-dimensional perovskite.

[0012] Preferably, the general formula of the two-dimensional perovskite single crystals is A'2B'X'4.

[0013] Wherein, A' is PEA + or FPEA + ; B' is Pb + , Sn + or Bi + ; X' is a halide anion.

[0014] Preferably, the concentration of AX in the AX ethanol solution is 0.02-1.5 mol / L.

[0015] Preferably, the voltage of the reverse bias electric field is uniformly and linearly increased in the range of 1-15 V; and the time of the first crystallization is ≤60 min.

[0016] Preferably, the voltage of the forward bias electric field is uniformly and linearly increased in the range of 1-15 V, and the time of applying the forward bias electric field is ≤10 min.

[0017] Preferably, the gradient perovskite heterojunction is a gradient perovskite heterojunction formed between two-dimensional perovskite and three-dimensional perovskite.

[0018] The general formula of the three-dimensional perovskite is ABX3.

[0019] Wherein, A is an organic cation or an inorganic cation; B is Pb + , Sn + or Bi + ; X is a halide anion.

[0020] The application further provides the gradient perovskite heterojunction prepared by the preparation method.

[0021] The application further provides application of the gradient perovskite heterojunction in the field of X-ray detection.

[0022] The application provides a preparation method of a gradient perovskite heterojunction, which comprises the following steps: adopting an electric field driven ion deposition method, under the condition that a reverse bias electric field is applied, ITO glass and ITO glass on which a perovskite single crystal is fixed are relatively arranged in an AX ethanol solution to perform first crystallization, a perovskite gradient layer is obtained, the reverse bias electric field is switched to a forward bias electric field to perform second crystallization, and the gradient perovskite heterojunction is obtained. A is an organic cation or an inorganic cation in the AX, and X is a halide anion in the AX. Since ion exchange usually occurs rapidly within a few seconds after the perovskite single crystal enters the AX ethanol solution, the reverse bias electric field is applied first to repel or delay the ion exchange process. In the process of crystallization, the perovskite single crystal has a positive charge and can repel all cations in the solution (as shown in the figure), the cations in the solution will be enriched at the end far away from the perovskite single crystal and gradually reduced at the end close to the perovskite single crystal, so that a perovskite gradient layer with gradually increasing cation content is formed in the direction away from the perovskite single crystal; and the problem that the cations combine with the inorganic framework (the anions left after the cations in the perovskite single crystal are dissolved in ethanol) too quickly, resulting in poor crystal quality, is avoided. Figure 2 In the application, in the process of the first crystallization, a new pure perovskite layer is not deposited on the surface of the perovskite single crystal (the new pure perovskite can be understood as being different from the perovskite of the perovskite single crystal), but a gradient layer of the new pure perovskite and the perovskite single crystal. At the same time, the negative charge of the ITO glass itself can attract cations, so that the cations will more tend to penetrate into the perovskite, that is, penetrate into the position of the ITO glass; since the cations will be enriched at the end close to the perovskite single crystal, the MA + cations with higher concentration can penetrate into the perovskite more deeply, which not only increases the thickness of the gradient layer of the new pure perovskite and the perovskite single crystal on the surface of the perovskite monomer, but also makes the new pure perovskite adhere to the two-dimensional perovskite firmly. + MA BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 It is a structural schematic diagram in the preparation process of the gradient perovskite heterojunction of the application.

[0024] Figure 2A schematic diagram of the preparation principle of the gradient perovskite heterojunction described in the application;

[0025] Figure 3 XRD diagrams of MAPbI3, (FPEA)2PbI4, the gradient perovskite heterojunction described in Example 1, and the perovskite heterojunction described in Comparative Example 1;

[0026] Figure 4 A schematic diagram of the structure of a tensile testing device;

[0027] Figure 5 The fastness test results of the gradient perovskite heterojunction described in Example 1;

[0028] Figure 6 SEM and EDS diagrams of the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Example 1;

[0029] Figure 7 A schematic diagram of the internal structure of the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Example 1;

[0030] Figure 8 A schematic diagram of the energy level structure of the gradient perovskite heterojunction described in Example 1;

[0031] Figure 9 The bulk resistivity of (FPEA)2PbI4 two-dimensional perovskite, MAPbI3 three-dimensional perovskite, and the gradient perovskite heterojunction described in Example 1;

[0032] Figure 10 The 1 / f noise of (FPEA)2PbI4 two-dimensional perovskite, MAPbI3 three-dimensional perovskite, and the gradient perovskite heterojunction described in Example 1;

[0033] Figure 11 The photoconductivity (μτ) test results of (FPEA)2PbI4 two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1;

[0034] Figure 12 The photoconductivity (μτ) test results of (FPEA)2PbI4 two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 at 500V·cm -1 On / off signal current response under a large electric field;

[0035] Figure 13 The SCLC test results of MAPbI3 three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1;

[0036] Figure 14 The sensitivity of (FPEA)2PbI4 two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 under different electric fields;

[0037] Figure 15 Current response of MAPbI3 three-dimensional perovskite and the gradient perovskite heterojunction described in Embodiment 1 under low dose rate X-ray of 28.2 nGy·s -1

[0038] Figure 16 Stability test of the gradient perovskite heterojunction described in Embodiment 1;

[0039] Figure 17 Single-pixel imaging system developed based on the gradient perovskite heterojunction described in Embodiment 1;

[0040] Figure 18 Actual image of the nut under light and imaging results of different radiation intensities by using the single-pixel imaging system described in Embodiment 1; Figure 17

[0041] Structural schematic diagram of the heterojunction device prepared from (FPEA)2PbI4 two-dimensional perovskite, MAPbI3 three-dimensional perovskite and the gradient perovskite heterojunction described in Embodiment 1; Figure 19

[0042] Actual image of the gradient perovskite heterojunction prepared in Embodiments 1-4. Figure 20 DETAILED DESCRIPTION

[0043] The present application provides a preparation method of a gradient perovskite heterojunction, comprising the following steps:

[0044] Under the condition of applying a reverse bias electric field, ITO glass and ITO glass fixed with perovskite single crystal are relatively arranged in AX ethanol solution to perform first crystallization, to obtain a perovskite gradient layer, and then the reverse bias electric field is switched to a forward bias electric field to perform second crystallization, to obtain the gradient perovskite heterojunction (as shown in Figure 1

[0045] A in AX is an organic cation or an inorganic cation, and X in AX is a halide anion.

[0046] In the present application, all the preparation raw materials are commercially available products well known to those skilled in the art, unless otherwise specified.

[0047] The present application does not have any special limitation on the spacing between the ITO glass and the ITO glass fixed with perovskite single crystal, which can be adjusted by using the spacing well known to those skilled in the art. In the embodiments of the present application, the spacing between the ITO glass and the ITO glass fixed with perovskite single crystal can be 1 cm.

[0048] ​​​In the present application, the perovskite single crystal fixed in the ITO glass is preferably two-dimensional perovskite.

[0049] In the present application, the general formula of the two-dimensional perovskite single crystal is preferably A'2B'X'4; wherein A' is preferably PEA + or FPEA + ; B' is preferably Pb + , Sn + or Bi + ; X' is preferably halogen anion, more preferably I - , Cl - or Br - .

[0050] In the present application, the preparation method of the ITO glass fixed with perovskite single crystal preferably comprises: fixing the perovskite single crystal on the ITO glass by using ultraviolet glue, and sealing. The present application does not have any special limitation on the type of the ultraviolet glue, which can be known by those skilled in the art. The present application does not have any special limitation on the process of sealing, which can be performed by using the process known by those skilled in the art. After the sealing is completed, the present application further preferably comprises increasing the electrical contact between the perovskite and the ITO glass by using gallium, so as to ensure that the perovskite crystal is in contact with the ITO, and to determine that the crystal can be powered on in the subsequent power-on step. In the present application, the sealing function is to prevent unnecessary contact of the solution with the edge of the crystal; the function of the gallium is to ensure that the electric field can be well applied to the entire perovskite crystal.

[0051] In the present application, the concentration of AX in the AX ethanol solution is preferably 0.02-1.5 mol / L, more preferably 0.3-1.2 mol / L. In the embodiment of the present application, the concentration of AX in the AX ethanol solution can be 0.4 mol / L.

[0052] In the present application, A in the AX is an organic cation or an inorganic cation, preferably an organic cation, more preferably MA + and / or FA + ; the inorganic cation is preferably Cs + ; X in the AX is a halogen anion, preferably I - , Cl - or Br - . In the present application, the A as an organic cation can better ensure the growth of the corresponding type of perovskite in the subsequent epitaxial layer.

[0053] In the present application, the value of the voltage can avoid the destruction of the epitaxial layer and the surface of the perovskite single crystal due to excessive voltage.

[0054] In the present application, the ion exchange process will generally occur rapidly within a few seconds after the perovskite single crystal enters the AX ethanol solution, so the application of the reverse bias electric field can repel or delay the ion exchange process.

[0055] In the present application, the first crystallization time is preferably ≤60 min, more preferably 30-50 min. In the present application, the first crystallization process preferably involves a uniform linear increase in the reverse bias electric field in the range of 1-15 V. In an embodiment of the present application, the first crystallization time can be 35 min, and the reverse bias electric field can increase at a rate of 0.067 V·s -1 .

[0056] In the present application, during the first crystallization process, the two-dimensional perovskite is positively charged and can repel all cations in the solution (as shown in Figure 2 ), which will be enriched at the end far from the perovskite and gradually reduced at the end close to the perovskite; thereby avoiding the problem of too fast combination of cations with the inorganic framework, resulting in poor crystal quality. In the present application, during the crystallization process, the deposition on the surface of the perovskite single crystal is not a new pure perovskite layer, but a gradient layer of new pure perovskite layer and perovskite single crystal (the gradient layer refers to the proportion of two kinds of perovskite in a certain layer gradually changes along the thickness direction).

[0057] In the present application, the voltage of the forward bias electric field preferably increases uniformly and linearly in the range of 1-15 V. The present application does not have any special limitation on the rate of increase of the reverse bias electric field. In the present application, the time for applying the forward bias electric field is preferably ≤10 min. In an embodiment of the present application, the time for applying the forward bias electric field can be 5 min, and the forward bias electric field can increase at a rate of 0.467 V·s -1 .

[0058] In the present application, during the application of the forward bias electric field, the cations in the solution will approach the perovskite single crystal substrate under the action of the electric field, and the forward bias electric field can further enhance the penetration ability of the cations to the lattice of the perovskite single crystal (as shown in Figure 2 ).

[0059] In the present application, the gradient perovskite heterojunction is preferably a gradient perovskite heterojunction formed between a two-dimensional perovskite and a three-dimensional perovskite; the general formula of the three-dimensional perovskite is preferably ABX3; wherein A is an organic cation or an inorganic cation, preferably an organic cation, more preferably MA + and / or FA + ; B is Pb + , Sn + or Bi +X is a halide anion, preferably I - , Cl - or Br - . In the present application, the general formula of the two-dimensional perovskite single crystal is preferably A'2B'X'4; wherein A' is preferably PEA + or FPEA + ; B' is preferably Pb + , Sn + or Bi + ; and X' is preferably a halide anion, more preferably I - , Cl - or Br - .

[0060] The present application also provides a gradient perovskite heterojunction prepared by the preparation method.

[0061] The present application also provides the use of the gradient perovskite heterojunction in the field of X-ray detection. The method for the use is not particularly limited in the present application, and can be performed by a method well known to those skilled in the art.

[0062] The gradient perovskite heterojunction, the preparation method and the use thereof provided by the present application will be described in detail below in conjunction with examples, but they should not be understood as limiting the scope of protection of the present application.

[0063] Comparative Example 1

[0064] A two-dimensional (FPEA)2PbI4 perovskite single crystal was fixed on ITO glass by using ultraviolet glue, and after sealing, the electrical contact between the (FPEA)2PbI4 perovskite single crystal and the ITO glass was enhanced by gallium to obtain ITO glass fixed with a perovskite single crystal.

[0065] The ITO glass fixed with the perovskite single crystal was immersed in an ethanol solution (concentration: 0.4 mol / L) of MAI for 60 min to obtain a gradient perovskite heterojunction (the internal structure is schematically shown in Figure 7 ).

[0066] Example 1

[0067] A two-dimensional (FPEA)2PbI4 perovskite single crystal was fixed on ITO glass by using ultraviolet glue, and after sealing, the electrical contact between the (FPEA)2PbI4 perovskite single crystal and the ITO glass was enhanced by gallium to obtain ITO glass fixed with a perovskite single crystal.

[0068] The ITO glass and the ITO glass with the perovskite single crystal fixed thereon were placed in relative positions (the distance between them was 1 cm) in the ethanol solution (the concentration was 0.4 mol / L) of MAI under the condition of applying a reverse bias electric field (the voltage of the reverse bias electric field was linearly increased at a uniform speed in the range of 1-15 V, the increasing rate was 0.067 V·s -1 After 35 min, the reverse bias electric field was switched to a forward bias electric field (the voltage of the forward bias electric field was linearly increased at a uniform speed in the range of 1-15 V, the increasing rate was 0.467 V·s -1 )5 min, to obtain the gradient perovskite heterojunction (the general formula of the three-dimensional perovskite in the gradient perovskite heterojunction formed between the two-dimensional perovskite and the three-dimensional perovskite was MAPbI3, the internal structure schematic diagram was as shown in Figure 7 It can be known from a of Figure 7 that the gradient perovskite heterojunction of Comparative Example 1 had a thinner gradient layer and was more prone to defects, and the gradient perovskite heterojunction of Example 1 firmly combined the three-dimensional perovskite and the two-dimensional perovskite together, thickened the gradient layer and optimized the energy band structure of the heterojunction; the energy level structure schematic diagram was as shown in Figure 8 It can be known from a of Figure 8 that when irradiated, electron-hole pairs were generated and separated under the action of the electric field. The holes flowed to the Au electrode through the valence band maximum (VBM) of the perovskite, and the electrons flowed to the Ga electrode through the conduction band minimum (CBM) of the 3D perovskite. The tilted valence band in the hierarchical heterojunction promoted the flow of electrons and holes in the wide depletion layer, enhanced the charge collection, and thus improved the charge separation and collection efficiency.

[0069] The actual diagram of the gradient perovskite heterojunction and the actual diagram under the excitation of a 532 nm ultraviolet flashlight were as shown in a of Figure 20 It can be known from a of Figure 20 that the actual diagram of the gradient perovskite heterojunction and the actual diagram under the excitation of a 532 nm ultraviolet flashlight were as shown in a of Figure 20 It can be known from a of Figure 20 that the gradient perovskite heterojunction appeared green light under the excitation of a 532 nm ultraviolet flashlight;

[0070] Figure 3 The XRD diagrams of MAPbI3, (FPEA)2PbI4, the gradient perovskite heterojunction of Example 1 and the perovskite heterojunction of Comparative Example 1 were as shown in Figure 3It can be seen that both the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Example 1 have characteristic peaks of MAPbI3(2q ~ 14.8°, 28.8°), confirming the presence of MAPbI3. However, compared with the gradient perovskite heterojunction described in Example 1, the perovskite heterojunction described in Comparative Example 1 obviously shows some impurity peaks, and exposes the characteristic peaks of (FPEA)2PbI4(2q ~ 21.8°, 27.2°), which is attributed to the fact that the EFID method used in Example 1 enhances the permeability of MA + ions and effectively increases the surface coverage of MAPbI3on the two-dimensional substrate; in addition, by adjusting the deposition rate by controlling the direction of the electric field, the crystal quality of MAPbI3is also improved, thereby reducing defects;

[0071] We evaluated the firmness of the heterojunction by using adhesive tapes with different viscosities, where the pulling force of the adhesive tape was measured using a spring tensile meter. During the test, one end of the crystal was adhered to the adhesive tape, while the other end was firmly fixed at the lower end of the spring tensile meter. When the spring tensile meter was slowly lifted, the force exerted on the crystal was roughly balanced. Considering that the weight of the crystal can be ignored, the pulling force of the adhesive tape on the bottom of the crystal will balance with the elastic force of the spring, and this force value will be displayed on the spring tensile meter (device schematic diagram is shown in Figure 4 ). Figure 5 The firmness test results of the gradient perovskite heterojunction described in Example 1 are shown in Figure 5 It can be seen that the heterojunction obtained by the EFID method (Example 1) does not detach even under an adhesive tape pulling force of about 10 N, indicating that it has high firmness. In contrast, the heterojunction obtained by the immersion method (Comparative Example 1) starts to detach at an adhesive tape pulling force of 3 N, and completely detaches at about 10 N. Therefore, the firmness of the heterojunction prepared by the electrodeposition method is superior to that of the heterojunction prepared by the immersion method;

[0072] Figure 6 The SEM and EDS images of the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Example 1 are shown in Figure 6It can be seen that the heterojunction surface prepared by the EFID strategy is more uniform and smooth, and the thickness of the 3D / 2D gradient layer is also larger after treatment with the same concentration of MAI solution, which can be directly observed from the SEM image. Fluorine element (F) is a characteristic element of organic cations in two-dimensional perovskite, and fluorine-lead ratio (F / Pb) and iodine-lead ratio (I / Pb) are used to jointly detect the diffusion of two-dimensional and three-dimensional structures. Compared with the simple immersion method, the F / Pb ratio and I / Pb ratio under the EFID strategy decrease more slowly on the 3D / 2D cross section, showing the characteristics of the gradient heterojunction. This gradient heterojunction can effectively increase the thickness of the depletion layer, improve charge separation and transmission, and suppress ion migration under a larger working bias. EDS scanning further shows that the thickness of the pure 3D perovskite prepared by the immersion method is much smaller than that obtained by the EFID strategy. This can be attributed to the limited ion diffusion capacity of the immersion method, which cannot provide sufficient driving force for ion exchange;

[0073] Figure 9 The heterojunction devices prepared by the (FPEA)2PbI4 two-dimensional perovskite, the MAPbI3 three-dimensional perovskite and the gradient perovskite heterojunction of Example 1 (the specific structure diagram is as shown in FIG. 1) Figure 19 , a is the heterojunction device prepared by the gradient perovskite heterojunction of Example 1 (including the ITO glass layer, the Ga layer, the gradient perovskite heterojunction layer (corresponding to the 2D perovskite layer and the 3D perovskite layer in the figure) and the Ag layer which are sequentially stacked), b is the heterojunction device prepared by the (FPEA)2PbI4 two-dimensional perovskite or the MAPbI3 three-dimensional perovskite (including the ITO glass layer, the Ga layer, the (FPEA)2PbI4 two-dimensional perovskite layer or the MAPbI3 three-dimensional perovskite layer (corresponding to the Control perovskite layer in the figure) and the Ag layer which are sequentially stacked), and the volume resistivity is Figure 9 It can be seen that the volume resistivity of the MAPbI3 three-dimensional perovskite is 1.60×1011Ω·cm; the volume resistivity of the gradient perovskite heterojunction of Example 1 is 8.84×10 10 Ω·cm, which is slightly lower, but still more than two orders of magnitude higher than that of the three-dimensional perovskite (5.59×10 8 Ω·cm).

[0074] Figure 10 The heterojunction devices prepared by the (FPEA)2PbI4 two-dimensional perovskite, the MAPbI3 three-dimensional perovskite and the gradient perovskite heterojunction of Example 1 (the specific structure diagram is as shown in FIG. 1) Figure 19 , a is the heterojunction device prepared by the gradient perovskite heterojunction of Example 1, b is the heterojunction device prepared by the (FPEA)2PbI4 two-dimensional perovskite or the MAPbI3 three-dimensional perovskite), and the 1 / f noise is Figure 10It can be seen that for X-ray detectors, higher volume resistivity means lower noise current, which improves the signal-to-noise ratio and provides a better detection limit. Therefore, its 1 / f noise is measured. Figure 10 It is confirmed that the ultra-low noise of the gradient perovskite heterojunction described in Example 1 is much lower than that of the three-dimensional perovskite;

[0075] Figure 11 The heterojunction device prepared by the (FPEA)2PbI4 two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Figure 19 , a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, b is the photoconductivity (μτ) test result of the heterojunction device prepared by (FPEA)2PbI4 two-dimensional perovskite). Figure 11 It can be seen that the μτ product of the gradient perovskite heterojunction in Example 1 is 1.21×10 -4 cm 2 ·V - 1, higher than the control device's 2.67×10 -5 cm 2 ·V - 1;

[0076] Figure 12 The heterojunction device prepared by the (FPEA)2PbI4 two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Figure 19 , a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, b is the heterojunction device prepared by the (FPEA)2PbI4 two-dimensional perovskite) at 500V·cm -1 The on / off signal current response under large electric field is given by Figure 12 It can be seen that the signal current of the gradient perovskite heterojunction described in Example 1 is significantly higher than that of the pure (FPEA)2PbI4 two-dimensional perovskite, which preliminarily confirms the results of the μτ product;

[0077] Defect density measurement is an important criterion for determining device noise in low-dose probing. We use the space charge limited current (SCLC) technique to evaluate the defect density of different devices. Figure 13 The heterojunction device prepared by the MAPbI3 three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Figure 19 , a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, b is the SCLC test results of the heterojunction device prepared by MAPbI3 three-dimensional perovskite); Figure 13 It can be seen that Figure 13The SCLC curve of the gradient perovskite heterojunction device shows three different regions, including Ohmic region, TFL region and Child region, which determine the defect density and carrier mobility. The defect density of the gradient perovskite heterojunction device described in Example 1 is 1.18 x 10 9 cm -3 , while the defect density of the three-dimensional perovskite device is 2.25 x 10 10 cm -3 , indicating the superiority of the heterojunction device in defect density. Although the carrier mobility of the heterojunction single crystal is about 7.4 cm 2 ·V -1 ·s -1 , which is lower than the expected 21.7 cm 2 ·V -1 ·s -1 of the three-dimensional perovskite, the signal-to-noise ratio of the heterojunction device is higher than that of the corresponding devices of two-dimensional and three-dimensional perovskites, thus having the potential to achieve higher signal-to-noise ratio and lower detection limit;

[0078] Figure 14 The sensitivity of the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1 (the specific structure is shown in Figure 19 , a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, and b is the heterojunction device prepared by the (FPEA)2PbI4 two-dimensional perovskite) under different electric fields, which is shown in Figure 14 It can be seen that the net response current is highly sensitive to the dose rate under different electric fields, and the response of the gradient perovskite heterojunction described in Example 1 is significantly higher than that of pure (FPEA)2PbI4 two-dimensional perovskite. By fitting the slope of the response current density and the applied dose rate, it is found that the sensitivity of the heterojunction single crystal detector under an electric field intensity of 500 V·mm -1 is as high as 21225 μC·Gy -1 ·cm -2 , which is 243% higher than that of (FPEA)2PbI4 two-dimensional perovskite (6717 μC·Gy -1 ·cm -2 ). This excellent X-ray sensitivity can be attributed to the enlarged built-in electric field width by the gradient heterojunction. Due to its high bulk resistivity and lower defect density, the perovskite heterojunction detector exhibits excellent detection limit and ion migration suppression capability;

[0079] Figure 15 The sensitivity of the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1 (the specific structure is shown in Figure 19, a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, b is the heterojunction device prepared by MAPbI3 three-dimensional perovskite) at 28.2nGy·s -1 The current response to low-dose-rate X-rays is given by Figure 15 It can be seen that even at a bias voltage of 50V, the MAPbI3 three-dimensional perovskite has almost no signal response, while the gradient perovskite heterojunction described in Example 1 can still generate significant signal current even at a high bias voltage of 200V;

[0080] Figure 16 The stability test of the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1 was conducted by Figure 16 It can be seen that the heterojunction device shows excellent stability in harsh external environments. -1 Under high electric fields, irradiated with 120kV hard X-rays with a total dose of 2.5Gy, it can still output stable signal current, which is significantly better than the three-dimensional perovskite control group under the same conditions;

[0081] Evaluation Figure 17 The X-ray imaging capability of the single-pixel imaging system (developed based on the gradient perovskite heterojunction described in Example 1) is such that the gradient perovskite heterojunction described in Example 1 acts as a sensing pixel and can be continuously moved in the XY direction. After recording the current value of each pixel, the computer generates and processes the image. Figure 18 The actual picture of the nut under the light and the Figure 17 The imaging results of the single-pixel imaging system at different ray intensities are given by Figure 18 It can be seen that at 578μGy·s -1 The device can obtain very clear images at higher dose rates. According to the definition of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the dose rate is less than 0.1mGy·min -1 (1.67μGy·s -1 ) is considered to be a low dose rate and will not cause substantial pathological harm to human health. Even if the dose rate is reduced to 8.57 μGy·s -1 and 1.31 μGy·s -1 , the outline of the nut can still be identified, indicating that this heterojunction device has potential in the field of low-dose imaging.

[0082] Example 2

[0083] Reference Example 1, except that the ethanol solution of MAI is replaced by an ethanol solution of FAI to obtain a gradient perovskite heterojunction; the actual photographs of the (FPEA)2PbI4 two-dimensional perovskite single crystal, the gradient perovskite heterojunction and the gradient perovskite heterojunction after annealing at 150℃ for 30min are shown in Figs. 1a, 1b and 1c, respectively. Figure 20 As shown in b of Fig. 1, at room temperature, the just generated FAPbI3 exhibits a yellow δ-FAPbI3 phase, also known as yellow phase, which is relatively stable but has poor performance. After annealing at 150℃, the yellow δ phase is converted into α phase, also known as black phase, which is the phase actually used in perovskite radiation detectors. This phase transition also proves that FAPbI3 is indeed generated. Figure 20

[0084] Example 3

[0085] Reference Example 1, except that the (FPEA)2PbI4 two-dimensional perovskite single crystal is replaced by MAPbBr3 crystals, and the ethanol solution of MAI is replaced by an ethanol solution of FAI to obtain a gradient perovskite heterojunction; the actual photographs of the MAPbBr3 crystals and the gradient perovskite heterojunction are shown in Figs. 2a and 2b, respectively. Figure 20

[0086] Example 4

[0087] Reference Example 1, except that the (FPEA)2PbI4 two-dimensional perovskite single crystal is replaced by CsPbBr3 crystals, and the ethanol solution of MAI is replaced by an ethanol solution of PEAI to obtain a gradient perovskite heterojunction; the actual photographs of the CsPbBr3 crystals, the gradient heterojunction and the gradient heterojunction under excitation of 532nm ultraviolet flashlight are shown in Figs. 3a, 3b and 3c, respectively. Figure 20 As shown in d of Fig. 3, (PEA)2PbBr4 emits blue light, while the substrate perovskite CsPbBr3 emits green light. Figure 20

[0088] The above only describes the preferred embodiments of the present application, and it should be noted that those skilled in the art can make several improvements and refinements without departing from the principles of the present application, and these improvements and refinements should also be considered as the protection scope of the present application.​​​

Claims

1. A method for preparing a gradient perovskite heterojunction, characterized in that: The following steps are involved: Under the condition of applying a reverse bias electric field, the ITO glass and the ITO glass with the perovskite single crystal fixed thereon are placed relative to each other in an ethanol solution of AX and then subjected to a first crystallization to obtain a perovskite graded layer. The reverse bias electric field is then switched to a forward bias electric field to perform a second crystallization to obtain the graded perovskite heterojunction. A in the AX is an organic cation or an inorganic cation, and X in the AX is a halogen anion.

2. The preparation method according to claim 1, wherein The organic cation is MA + and / or FA + ; The inorganic cation is Cs + .

3. The preparation method according to claim 2, wherein The perovskite single crystal in the ITO glass fixed with the perovskite single crystal is a two-dimensional perovskite.

4. The preparation method according to claim 1 or 3, wherein The general formula of the perovskite single crystal is A'2B'X'4; Where A' is PEA + or FPEA + ; B' is Pb + 、Sn + Or Bi + ; X' is a halogen anion.

5. The preparation method according to claim 1, wherein The concentration of AX in the ethanol solution of AX is 0.02-1.5 mol / L.

6. The preparation method according to claim 1, wherein The voltage of the reverse bias electric field increases linearly and uniformly within the range of 1 to 15 V; and the time of the first crystallization is ≤ 60 min.

7. The preparation method according to claim 1, wherein The voltage of the forward bias electric field increases linearly and uniformly within the range of 1 to 15 V, and the time for applying the forward bias electric field is ≤ 10 min.

8. The preparation method according to any one of claims 3, 5 to 7, wherein The gradient perovskite heterojunction is a gradient perovskite heterojunction formed between a two-dimensional perovskite and a three-dimensional perovskite; The general formula of the three-dimensional perovskite is ABX3; Wherein, A is an organic cation or an inorganic cation; B is Pb + 、Sn + Or Bi + ; X is a halogen anion.

9. The gradient perovskite heterojunction prepared by the preparation method according to any one of claims 1 to 8.

10. Application of the gradient perovskite heterojunction according to claim 9 in the field of X-ray detection.

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